Secondary battery comprising a polymerizable material in its electrolyte solution

ABSTRACT

A battery excellent in high temperature storage characteristic is presented. It comprises a positive electrode having a positive electrode active material containing an transition metal complex oxide containing lithium , a negative electrode containing a negative electrode material capable of storing and releasing a lithium ion, and an electrolytic solution containing a nonaqueous solvent, an electrolyte, and an organic compound expressed in formula 1. ##STR1## where R 1 , R 2 , R 3 , R 4 , R 5 , and R 6  have individually at least one of H and a group containing a vinyl group, and the number of H substituent is four or less.

FIELD OF THE INVENTION

The present invention relates to a secondary battery, and more particularly to a secondary battery using a nonaqueous electrolytic solution.

BACKGROUND OF THE INVENTION

Recently, the portable and cordless trend is rapidly promoted in electronic appliances such as AV appliances and personal computers. As the driving power source for these electronic appliances, there is a mounting demand for secondary batteries having high energy density in spite of small size and light weight. In particular, the nonaqueous electrolyte secondary battery with negative electrode using lithium as active material is highly expected as a battery having high voltage and high energy density.

In this battery, the active material for the positive electrode is a metal oxide containing lithium showing a voltage in a class of 4 V such as LiCoO₂, LiNiO₂, or LiMn₂ O₄, and the negative electrode is a carbon material which is capable of intercalating and deintercalating lithium.

The electrolytic solution to be used in such nonaqueous electrolyte battery is, for example, a nonaqueous solvent such as ethylene carbonate, propylene carbonate or other cyclic ester, tetrahydrofuran or other cyclic ether, dimethoxy ethane or other noncyclic ether, dimethyl carbonate, methyl propionate or other noncyclic ester, or a mixed solvent thereof.

Lately, the secondary batteries are expected to be used in various applications, not only in portable data terminals (for example, cellular phones and notebook type personal computers), but also in electric vehicles and power storage. Accordingly, applications in severe environments are increasing. In particular, when used as the power source for an electric vehicle, the battery may be exposed to high temperature of over 80° in summer, and the battery having a high reliability even in such severe ambient temperature is demanded.

In these secondary batteries, accordingly, a nonaqueous organic solvent not electrochemical-decomposed at high voltage is used. Such batteries are particularly heightened in oxidation catalytic function in the positive electrode in high temperature environment. As a result, the conventional nonaqueous solvent is electrochemical oxidized on the surface of the positive electrode, the discharge characteristic deteriorates due to drop of conductivity of the electrolytic solution, and decomposition products (for example, carbon dioxide) are formed as gas. In a worst case, consequently, leakage of electrolytic solution occurred.

It is hence an object of the invention to present an excellent battery having a high reliability even when stored at high temperature.

SUMMARY OF THE INVENTION

The secondary battery of the invention comprises a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode material, and an electrolyte solution containing a nonaqueous solvent, an electrolyte, and an organic compound expressed in formula 1. ##STR2##

In formula 1, each of R₁, R₂, R₃, R₄, R₅, and R₆ has at least one of H and a group containing a vinyl group, and the number of H substituent is four or less.

Preferably, the positive electrode active material is an transition metal complex oxide containing lithium , and the negative electrode material has a material capable of storing and releasing lithium.

Preferably, the organic compound has at least one selected from the group consisting of o-divinyl benzene, m-divinyl benzene, p-divinyl benzene, and 1,3,5-trivinyl benzene.

Preferably, the organic compound is contained in a range of about 0.1 wt. % to about 20 wt. % in the total weight of the electrolytic solution.

In this constitution, the resistance of the electrolytic solution to oxidation is enhanced. Accordingly, deterioration of battery is prevented even when stored in high temperature atmosphere. As a result, a battery having a high reliability is obtained.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal sectional view of a secondary battery in an embodiment of the invention.

    ______________________________________                                         REFERENCE NUMERALS                                                             ______________________________________                                                  1   Battery case                                                               2   Sealing plate                                                              3   Insulating packing                                                         4   Plate group                                                                5   Positive electrode lead                                                    6   Negative electrode lead                                                    7   Insulating ring                                                   ______________________________________                                    

DETAILED DESCRIPTION OF THE INVENTION

The following have been disclosed as a result of close examination about behavior of electrochemical oxidation of electrolytic solution. Electrochemical Oxidation occurs on the surface of a transition metal complex oxide containing lithium used as active material for positive electrode. The decomposed product contains lots of low molecular compounds. Such low molecular compounds are released outside of the electrolytic solution as gas, or dissolved in the electrolytic solution. When oxidation takes place in such manner, active center of the positive electrode active material maintain a high activity without being covered. Accordingly, the decomposition reaction of oxidation of the electrolytic solution progresses continuously, and the electrolytic solution is exhausted, or decomposition gas is generated massively, which may possibly lead to elevation of internal pressure of the battery or leakage of electrolytic solution.

In the secondary battery of the invention, an organic compound expressed in formula 1 is added in the electrolytic solution. Such organic compound is likely to be oxidized and polymerized. By using the electrolytic solution containing such organic compound, a protective film is formed at active center of the positive electrode surface, and continuous oxidation of the electrolytic solvent is prevented. ##STR3## where R₁, R₂, R₃, R₄, R₅, and R₆ are individually at least one of H and a group containing a vinyl group, and the number of H substituent is four or less.

The organic compound expressed in formula 1 has plural vinyl groups bonded by benzene skeleton, and has a high pi-electron density. Such organic compound is likely to be oxidized and polymerized, and the product of oxidation and polymerization can cover the active center of the positive electrode effectively.

Preferably, the constitution is specified below.

The active material for positive electrode of transition metal complex oxide containing lithium includes Li_(x) CoO₂, Li_(x) NiO₂ (U.S. Pat. No. 4302518), Li_(x) MnO₂, Li_(x) Co_(y) Ni_(1-y) O₂ (Japanese Laid-open Patent No. 63-299056), Li_(x) Co_(f) V_(1-f) O_(z), Li_(x) Ni_(1-y) M_(y) O₂ (M=Ti, V, Mn, Fe), Li_(x) Co_(a) Ni_(b) M_(c) O₂ (M=Ti, Mn, Al, Mg, Fe, Zr), Li_(x) Mn₂ O₄, and Li_(x) Mn_(2-y) M_(y) O₄ (M=Na, Mg, Sc, Y, Fe, Co, Ni, Ti, Zr, Cu, Zn, Al, Pb, Sb). Herein, x=0 to 1.2, y=0 to 0.9, f=0.9 to 0.98, z=2.0 to 2.3, a+b+c=1, 0.a.1, 0.b.1, 0.c<1. The value of x is the value before start of discharging and charging , and the value of x increases or decreases by charging and discharging.

The negative electrode material is a compound capable of storing and releasing lithium ions, such as lithium, lithium alloy, alloy, intermetallic compound, carbon, organic compound, inorganic compound, metal complex, and organic high molecular compound. These substances are used either alone or in mixture.

As the alloy or intermetallic compound, a compound of transition metal and silicon, or a compound of transition metal and tin is used. In particular, a compound of nickel and silicon is preferable.

Examples of carbonaceous material include coke, pyrolytic carbons, natural graphite, artificial graphite, mesocarbon microbeads, graphitized mesophase spherule, vapor phase growth carbon, glassy carbon, carbon fiber (polyacrylonitrile, pitch, cellulose, vapor phase growth carbon), amorphous carbon, and organic matter baked carbon. These materials are used either alone or in mixture. In particular, graphite materials are preferred, such as graphitized material of mesophase spherule, natural graphite and artificial graphite. As carbonaceous materials, in addition to carbon, a material containing different compounds such as O, B, P, N, S, SiC, and B4C are used, and the content of such different compounds is preferred to be 0 to 10 wt. %.

As the inorganic compound, for example, tin compound or silicon compound is used.

The mean particle size of carbon material is preferred to be in a range of about 0.1 μm to 60 μm, and more preferably about 0.5 μm to about 30 μm. The specific surface area is preferred to be 1 m² /g to 10 m² /g. As a preferred crystal structure of graphite, the interval (d002) of hexagonal planes of carbon is 3.35 Å to 3.40 Å, and the size (Lc) of crystallite in the c-axis direction is 100 Å or more.

The electrolytic solution contains nonaqueous solvent and lithium salt to be dissolved in this solvent. As the nonaqueous solvent, cyclic carbonate, noncyclic carbonate, aliphatic ester carbonate, and others may be used. Examples of cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC). Examples of noncyclic carbonate include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dipropyl carbonate (DPC). Examples of aliphatic ester carbonate include methyl formate, methyl acetate, methyl propionate, and ethyl propionate. More preferably, the nonaqueous solvent is mainly composed of a solvent containing at least cyclic carbonate, a mixed solvent of cyclic carbonate and noncyclic carbonate, or a mixed solvent of cyclic carbonate, noncyclic carbonate, and aliphatic ester carbonate.

As the electrolyte to be dissolved in these solvents, lithium salt such as LiPF₆ is preferred. The dissolving amount of lithium salt in the nonaqueous solvent is not particularly specified, but it is preferably about 0.2 mol/liter to about 2 mol/liter, and more preferably about 0.5 mol/liter to about 1.5 mol/liter.

As the positive electrode active material, at least one may be used as being selected from the group consisting of Li_(x) CoO₂, Li_(x) NiO₂, and Li_(x) Mn₂ O₄ (where 0≦x≦1). As the positive electrode current collector, a net, sheet, foil, lath or other shape made of stainless steel or aluminum may be used.

As the negative electrode material, instead of lithium metal alone, a compound containing at least one of alloy and carbonaceous material and lithium is used. As the negative electrode current collector, a net, sheet, foil, lath or other shape made of stainless steel or copper may be used.

As the electrode compound containing positive electrode active material or negative electrode material, binder, an electron conductor such as acetylene black or graphite is mixed. Examples of binder include polyvinylidene fluoride, polytetrafluoroethylene or other thermoplastic compound containing fluorine, polymer containing acrylic acid, styrene butadiene rubber, ethylene propylene terpolymer, or other elastomer, which may be used either alone or in mixture.

Preferably, the electrolytic solution contains at least (a) ethylene carbonate, (b) at least one of noncyclic carbonates such as diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate, or a mixed solvent of such noncyclic carbonate and methyl propionate, and (c) LiPF₆ as lithium salt. As the separator, polypropylene or polyethylene may be used either alone or in combination.

The shape of the battery is not particularly limited, but it may be formed in cylindrical, flat, prismatic, square or other shape. Preferably, the battery has means for assuring safety even in case of malfunction. The safety assuring means may include, for example, internal pressure relief safety valve, current shut-off safety valve, and separator for raising resistance at high temperature.

Referring now to the drawing, a typical embodiment of the invention is described below.

(Exemplary embodiment 1)

FIG. 1 is a longitudinal sectional view of a secondary battery in an embodiment of the invention. In FIG. 1, the secondary battery comprises a battery case 1, a sealing plate 2 having a safety valve, an insulating packing 3, a plate group 4, two insulating rings 7, a positive electrode lead 5, and a negative electrode lead 6. The battery case 1 is manufactured by processing a stainless steel plate having resistance to organic electrolytic solution. The sealing plate 2 is disposed so as to cover the opening of the battery case 1. The plate group 4 has a positive electrode and a negative electrode wound spirally plural times through a separator, and is contained in the case 1. The positive electrode lead 5 is drawn out of the positive electrode, and is connected to the sealing plate 2. The negative electrode lead 6 is drawn out of the negative electrode, and is connected to the bottom of the battery case 1. The first insulating ring 7 is disposed in the upper part of the plate group 4, and the second insulating ring 7 is disposed in the lower part of the plate group 7.

The positive electrode is manufactured in the following process.

(a) A mixture of Li₂ CO₃ and Co₃ O₄ is baked at 900° C. for 10 hours, and LiCoO₂ powder is synthesized.

(b) A mixture is prepared by mixing LiCoO₂ powder, 3% of acetylene black, and 7% of binder made of fluorocarbon resin.

(c) The mixture is suspended in an aqueous solution of carboxy methyl cellulose, and positive electrode compound paste is prepared.

(d) This positive electrode compound paste is applied on an aluminum foil of 0.03 mm in thickness, and is dried and rolled.

In this process, a positive plate of 0.18 mm in thickness, 37 mm in width, and 390 mm in length was fabricated.

The negative electrode is manufactured in the following process.

(a) Mesophase graphite is prepared by graphitizing mesophase spherule at high temperature of 2800° C.

(b) A mixture is prepared by mixing 5 wt. % of styrene/butadiene rubber into this mesophase graphite.

(c) This mixture is suspended in an aqueous solution of carboxy methyl cellulose, and negative electrode compound paste is prepared.

(d) This negative electrode compound paste is applied on both sides of a Cu foil of 0.02 mm in thickness, and dried and rolled.

Thus, a negative plate of 0.20 mm in thickness, 39 mm in width, and 420 mm in length was fabricated.

An aluminum lead is attached to the positive plate, and a nickel lead to the negative plate. The positive plate and negative plate were spirally wound through a polypropylene separator of 0.025 mm in thickness, 45 mm in width, and 950 mm in length, and put in a battery case 1 of 17.0 mm in diameter and 50.0 mm in height.

The electrolytic solution was prepared in the following process.

(a) A mixed solvent is prepared by mixing 30 vol. % of ethylene carbonate, 50 vol. % of diethyl carbonate, and 20 vol. % of methyl propionate.

(b) In this mixed solvent, LiPF₆ is dissolved by 1 mol/liter.

(c) Further, an organic compound expressed in formula 1 is added by a rate of 2 wt. % to the total weight of the electrolytic solution. As the organic compound expressed in formula 1, one of o-divinyl benzene, m-divinyl benzene, p-divinyl benzene, and 1,3,5-trivinyl benzene is used. In this way, four kinds of electrolytic solution were prepared.

Each one of these electrolytic solutions was poured into the battery case 1 containing the plate group 4, and the opening was sealed.

Thus, four batteries, No. 1 to No. 4, were fabricated.

(Comparative example)

By using the electrolytic solution not containing the organic compound expressed in formula 1, battery No. 5 was prepared in the same method as in the embodiment. This is a comparative battery 5.

The following tests were conducted by using five cells each of batteries No. 1 to No. 4 of the embodiment and No. 5 of comparative battery. Each cell was charged at constant voltage of charging voltage of 4.2 V, and limiting current of 500 mA for 2 hours of charging time at the ambient temperature of 20. In thus charged cells, the discharge characteristic at 1 A was measured. The capacity was measured. Later, these cells were stored in a thermostatic oven at 80° in charged state for 15 days. Then, the stored cells were charged and discharged in the same condition as before storage, and the capacity after storage was measured. Thus, the capacity recovery rate after storage was calculated. The results are recorded in Table 1. The capacity recovery rate (or capacity maintenance rate) after storage was calculated in the following formula.

Capacity recovery rate (%)=(capacity after storage/capacity before storage)×100

                  TABLE 1                                                          ______________________________________                                                  Organic     Recovery rate                                             o.       compound added                                                                             after storage                                             ______________________________________                                                  o-divinyl   90.3                                                               benzene                                                                        m-divinyl   92.8                                                               benzene                                                                        p-divinyl   91.9                                                               benzene                                                                        1,3,5-trivinyl                                                                             93.9                                                               benzene                                                                        None        65.2                                                      ______________________________________                                    

The following are disclosed from Table 1. The batteries of the embodiment, No. 1 to No. 4, had the capacity recovery rate (capacity maintenance rate) of over 90%. By contrast, battery No. 5 not containing the organic compound in formula 1 had the capacity recovery rate of 65.2%. Thus, the batteries using the electrolyte solution containing the organic compound in formula 1 are known to have an excellent capacity recovery rate (capacity maintenance rate).

A next test was conducted to investigate the effects of the rate of content of the organic compound in formula 1. Using the same electrolytic solutions as in the embodiment, various batteries were fabricated. In this case, the contents of the organic compounds specified in formula 1 in the total weight of the electrolytic solution were varied in a range of 0.05 wt. % to 30 wt. %. Using such batteries, the same test as in the embodiment was conducted. As a result, the batteries using the electrolytic solution with the content of organic compound of 0.1 wt. % and higher are known to have a particularly excellent capacity recovery rate (capacity maintenance rate) after storage. However, the batteries using the electrolytic solution containing the organic compound by over 20 wt. % were lowered in the discharge characteristic of the battery. This is considered because the electric conductivity of the electrolytic solution itself was decreased. That is, when the organic compound expressed in formula 1 is contained in a range of about 0.1 wt. % to 20 wt. % in the total weight of the electrolytic solution, batteries having a particularly excellent high temperature storage characteristic are obtained.

As described herein, according to the constitution of the invention, batteries having excellent characteristics are obtained even when stored in high temperature atmosphere. 

What is claimed is:
 1. A secondary battery comprising:a positive electrode comprising a positive electrode active material; a negative electrode comprising a negative electrode material; and an electrolyte solution comprising a nonaqueous solvent, an electrolyte, and an organic compound expressed in formula 1: ##STR4## in which each of R₁, R₂, R₃, R₄, R₅, and R₆ is either H or a group containing a vinyl group, and the number of H substituents is four or fewer.
 2. The secondary battery of claim 1, wherein:said positive electrode active material comprises a transition metal complex oxide that comprises lithium, and said negative electrode material comprises a material capable of storing and releasing lithium ions.
 3. The secondary battery of claim 2, wherein said organic compound is selected from the group consisting of o-divinyl benzene, m-divinyl benzene, p-divinyl benzene, and 1,3,5-trivinyl benzene.
 4. The secondary battery of claim 3, wherein said electrolyte comprises a lithium salt.
 5. The secondary battery of claim 4, wherein said organic compound comprises about 0.1 wt. % to about 20 wt. % of said electrolyte solution.
 6. The secondary battery of claim 5, wherein said lithium salt is LiPF₆.
 7. The secondary battery of claim 1,wherein said organic compound is at least one selected from the group consisting of o-divinyl benzene, m-divinyl benzene, p-divinyl benzene, and 1,3,5-trivinyl benzene.
 8. The secondary battery of claim 1, wherein said organic compound comprises about 0.1 wt. % to about 20 wt. % of said electrolyte solution.
 9. The secondary battery of claim 1,wherein said positive electrode active material comprises at least one transition metal complex oxide selected from the group consisting of Li_(x) CoO₂, Li_(x) NiO₂, Li_(x) MnO₂, Li_(x) Co_(y) N_(1-y) O₂, Li_(x) Co_(f) V_(1-f) O_(z), Li_(x) Ni_(1-y) M_(y) O₂ (M=Ti, V, Mn, Fe), Li_(x) Co_(a) Ni_(b) M_(c) O₂ (M=Ti, Mn, Al, Mg, Fe, Zr), Li_(x) Mn₂ O₄, and Li_(x) M_(2-y) M_(y) O₄ (M=Na, Mg, Sc, Y, Fe, Co, Ni, Ti, Zr, Cu, Zn, Al, Pb, Sb); in which:x=0 to 1.2, y=0 to 0.9, f=0.9 to 0.98, z=2.0 to 2.3, a+b+c=1, 0≦a≦1, 0≦b≦1, 0≦c<1, and the value of x is the value before start of charging and discharging, and the value of x decreases or increases by charging and discharging.
 10. The secondary battery of claim 1, whereinsaid negative electrode material comprises a compound capable of storing and releasing lithium ions.
 11. The secondary battery of claim 1,wherein said nonaqueous solvent contains at least one selected from the group consisting of (a) cyclic carbonate, (b) cyclic carbonate and noncyclic carbonate, and (c) cyclic carbonate, noncyclic carbonate, and fatty acid ester carbonate, said cyclic carbonate contains at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate, said noncyclic carbonate contains at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate, and said fatty acid ester carbonate contains at least one selected from the group consisting of methyl formate, methyl acetate, methyl propionate, and ethyl propionate.
 12. The secondary battery of claim 1, wherein said electrolyte comprises a lithium salt.
 13. A secondary battery comprising:(a) a positive electrode active material comprising Li_(x) CoO₂, in which x is 0 to 1.2; (b) a negative electrode material comprising graphite; and (c) an electrolyte solution comprising LiPF₆, o-divinyl benzene, ethylene carbonate, and a noncyclic carbonate; wherein said noncyclic carbonate comprises at least one noncyclic carbonate selected from the group consisting of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.
 14. The secondary battery of claim 13, wherein said o-divinyl benzene compound comprises about 0.1 wt. % to about 20 wt %. of said electrolyte solution. 